There isn't one..
Your best bet is, take a mainline train to Toulouse. (fairly flat country, lots of vineyards). Then a branch-line to "l'Hospitalet-pres-de-l'Andorre". Keep …your eyes open for Pamiers (birth-place of composer Gabriel Faure), Foix (interesting chateau), and some rugged scenery after Foix..
At l'Hospitalet you are a few miles from Andorra. Uphill all the way ! There is a rather irregular bus service; a taxi might be better. (MORE)